The most severely injured victims of the July 7 suicide bombings in London could be left struggling financially for the rest of their lives because of the shortcomings of the government's criminal injuries compensation scheme, lawyers have told the Guardian.

Some of the most badly hurt survivors of the terrorist attacks, which killed 52 people and injured more than 700, have told solicitors they are concerned about their financial future and are angry at the long delays they face - up to 15 months and more - before they receive a penny.

They are struggling to pay bills because they have not returned to work as a result of their injuries and psychological trauma. Some victims who lost limbs face bills of £20,000 for specialist prosthetics which are not available on the NHS. They will also need hundreds of thousands of pounds for the rest of their lives for their ongoing care. Many will not be able to return to the highly paid jobs they held before the attack and the concerns about their financial future are adding to their stress.

Under the government's criminal injuries compensation scheme, payouts range from £1,000 to £250,000. In the most severe cases the board may consider adding compensation for loss of earnings and medical costs but the total payout is capped at £500,000.

Personal injury experts say these record damages, which are given in a very few cases, will not cover the loss of earnings, equipment and care costs needed for a young victim who has suffered serious injuries in the terrorist attack.

Colin Ettinger, a partner at Irwin Mitchell solicitors, which has 14 clients who were involved in the bombings, said: "The shortfall will be monumental. The other key thing is that people really need the money now and they really need interim payments from the criminal injuries compensation board of some substance."

Sally Moore, head of personal injuries at Leigh Day, which represents six clients involved in the bombings, said many of the most badly injured were people in their late 20s or early 30s.

She said that even if people were awarded the full amount of £500,000 it would not touch the surface of their claims. She compared the awards to civil settlements of more than £1m for victims of the Paddington rail crash.

"It's a significant issue," said Ms Moore. "There will be plenty of cases arising out of the bombings that will far exceed the cap. There are cases of traumatic amputation from the bombings and there will be head injury cases where probably people will not be cognitiviely working at the same level they did before.

"What it means is you are worse off if you are a victim of terrorism than if you were injured in a road accident or rail crash."

Victims are also receiving money from the London Bombings Relief Charitable Fund - known as the Mayor's Fund - which has raised £8.77m from public donations. But the fund, administered by the Red Cross, has yet to pay out any substantial grants to survivors. It announced yesterday that the most seriously injured were likely to receive £15,000 - an amount condemned by some badly injured survivors.

"How have they calculated this? I think this is disgusting," said one victim who lost both legs in the bombings. "We really need money now, substantial amounts. The amounts they are giving could probably be paid out of the interest on the £8m they have raised."